Various expandable implantable medical devices such as stents, bone clips, vena cava filters, etc., are most easily and safely inserted into a passageway of the body such as the vascular system if they are first compressed into a small diameter configuration, then inserted into the body and expanded. Stents, for example, are compressed to fit onto or into a delivery catheter, which is then inserted into the body vessel such as a coronary artery, then expanded and released or released and expanded.
Because the catheter and stent must travel through the patient's vasculature, the stent must have a small delivery diameter and must be firmly secured within a delivery catheter until the physician is ready to implant it. Thus, the stent must be loaded onto or into the delivery catheter so that it does not interfere with delivery, and it must not come off the catheter until it is implanted.
In procedures where a self-expanding medical device such as a stent is utilized, the stent is placed within a protective delivery sleeve of the delivery catheter. It is necessary to properly collapse or compress the stent for loading it into the protective delivery sleeve. This collapsing or compressing of the stent has proven to be a particular challenge where it is necessary to load the stent into a small diameter delivery catheter.
In one loading procedure, a self-expanding stent is compressed to a small diameter and then inserted into a transfer tube. The compressed self-expanding stent is then loaded into a delivery sleeve of the delivery catheter by pushing the compressed self-expanding stent from the transfer tube into the protective delivery sleeve. This loading procedure is completed by connecting a tapered dilator head to the distal end of the delivery catheter by, for example, positioning the distal tip on a central rod or tube that extends through the longitudinal passageway of the loaded stent.
Several commercially available stent crimping or compression devices are available, which uniformly compress a self-expanding stent from a fully expanded state to a much smaller diameter compressed state for insertion into a transfer tube or delivery catheter.
Many vascular medical procedures further require the placement of a coated implantable medical device into the body of the patient. The placement of a metal or polymeric device however, gives rise to numerous complications. In particular, the placement of a bare expandable, implantable medical device such as a stent causes trauma to the vascular wall resulting often times in hyperplasia and restenosis of the vessel with the proliferation of smooth muscle cells at the implantation site.
One approach to reducing the potential harmful effects of such an introduction is to provide a coated, expandable, implantable medical device such as a stent having a coating of a bioactive material such as an antiproliferative that is delivered with the stent at the implantation site. By so doing, the harmful effects associated with implantation can be diminished. In particular, bioactive compounds such as paclitaxel and other antiproliferative materials are applied to the surface of the stent. These antiproliferative materials come in contact with the vessel wall and inhibit, among others, hyperplasia and restenosis of the vessel.
This bioactive coating material along with the stent must be reduced to a small diameter for loading onto a delivery catheter. This loading procedure is often further complicated when the stent such as a nitinol stent must be cooled to temperatures below the martensitic start and final temperatures of the nitinol material prior to compression and insertion into the delivery catheter. This is done to reduce the force necessary to load nitinol stents into an insertion or delivery catheter. Because the nitinol stent is completely martensitic, it is pliable and ductile, and easily compressed for loading into a delivery catheter. However, cooling a nitinol stent with a bioactive material coating thereon presents additional loading problems.
Nitinol stents are typically placed in a cooling chamber or container in which the temperature of the stent is reduced to below its martensitic final temperature such as, for example, below −40 degrees C. After the fully expanded nitinol stent is cooled to below its martensitic final temperature, it is then positioned in a stent crimper or compression apparatus and uniformly compressed to a much smaller diameter size for loading into the delivery sheath of a delivery catheter. However, the stent compression apparatus is typically at a temperature below the austenitic start temperature of the nitinol stent. Depending upon the environmental conditions of the cooling chamber, water or ice as well as other condensates or sublimates, often form on the compression apparatus as well as the coated nitinol stent. This condensation or sublimates can adversely affect the operation of the compression device as well as form an additional layer or thickness on the surface of the coated nitinol stent. The condensate or sublimate can significantly affect the compression of the stent so that the coated stent cannot be easily pushed into a transfer tube or a delivery sheath of the delivery catheter. The condensate or sublimate can be water or ice crystals formed from water vapor or crystals formed from the sublimation of carbon dioxide gas. Other condensates and sublimates such as oxygen and other contaminants found, in, for example, the air, are contemplated.